The Freezing Point Depression

In my Current Topics class recently, we have been preparing a demonstration of a scientific topic and a lab activity that can go along with it. My group’s topic is the freezing point depression. It may not sound that complicated, and it is actually is rather simple, although it took much discussion, arguing, and questioning to figure out exactly how the freezing point depression works. This has been a topic of interest for my group over the past couple of weeks and turned out to be quite fascinating.

Depressing the freezing point or melting point of a substance, since the temperature below which a substance freezes and above which a substance melts is the same, simply refers to the process of lowering the temperature at which a solid can melt into a liquid. This can be done with the addition of salt. We found a fun experiment to do, which was the “Ice Cream Lab”, which was supposed to demonstrate the freezing point depression. Although it took us a while to figure out how it was being exemplified in the experiment, eventually we came to a conclusion. Basically, the ice cream ingredients are put in a bag that is placed inside a larger bag containing a salt and ice mixture. The salt allows the ice to remain at its normal temperature (below 0°C), but melt into its liquid form. This salt and ice brine that surrounds the ice cream ingredients allows the cream to freeze properly and uniformly, therefore, helping in the process of making homemade ice cream.

Even though we liked this lab activity, we thought that it was very important to see if and how this principle could relate to the real world and be applied to our daily lives. We researched and discovered that salt is put on roads in the winter season for the same purpose, and based on the same principle. Since Winnipeg is very cold, snowy, and icy, for at least 4 months of the year, the use of salt on the roads in very significant to daily life in the city. When the salt is put on the ice in winter, it lowers the melting point of the ice, so that the ice is able to melt into liquid water and run off the roads. Because the roads in Winnipeg get so icy and slippery in the winter and create dangerous driving conditions, the principle of the freezing point depression is essential for keeping the people out of hazardous situations.

I think asking and being critical of why something is happening is important to growing and understanding how the world works. We decided that in order to continue with this lab, we needed to understand why salt was able to depress the freezing point of ice. The answer has to do with the structure of water at a molecular level. Water is made up of H₂O molecules, and the positively charged hydrogen atoms of the water molecules are attracted to the negatively charged oxygen atoms of other water molecules, in a process called hydrogen bonding. In liquid water, because there is more heat energy, the molecules are able to move faster and flow past each other more easily. Similarly, as the temperature of water decreases, the rate of movement of the H₂O molecules is slowed down. Because of this reduction in speed of the moving molecules, the positive hydrogen atoms and negative oxygen atoms bond more firmly to one another and create a crystallized structure (ice). In simpler words, when salt is added to ice water, the salt particles disrupt the bonding process of the water molecules, allowing the substance to remain in liquid form at a temperature where it would normally be a solid. Because of the addition of salt, the molecules that would normally attract each other and bond firmly together at temperatures below 0°C, are interrupted in the process, decreasing the number of bonds formed and therefore, keeping the liquid from entering into a solid state.

I hope you learned something new about how salt can be used to depress the melting point or freezing point of a substance. If you are looking for an experiment that deals with the freezing point depression, an ice cream lab is a fun way to learn about it! Here is a link to one possible ice cream lab you could perform to become more knowledgeable on the subject and see the freezing point depression in action: http://www2.waterforduhs.k12.wi.us/staffweb/smith/Nancy/chemistry/ice%20cream%20lab.htm

Thank you.